I've used the XSM10K/10K as an interstage quite a few times, but not the 10K/15K. If you pull less then 2ma DC through the primary of the 10K/10K you can get away with series use, otherwise you need to parallel feed the interstage. If you use the 6SN7 I expect you will have to parallel feed it. You can use a CCS, resistor or a plate choke on the 6SN7 plate. Sounds much better then the cost suggests.
Matt